Quantifying the saltmarsh vegetation canopy
Recent concern over the possibility of increased flood risk (as a result of global and accompanying regional climatic changes) on the low-lying coasts of North-east Europe (including, e.g., the UK East coast) requires a better understanding of the natural sea-defence capacity of intertidal areas. While previous studies (e.g. Möller and Spencer (2003), Möller et al. (2001)) indicate that such environments can reduce flood risk by significantly attenuating waves over hydraulically rough vegetated saltmarsh surfaces (see also: Wave dynamics over saltmarsh surfaces), no study has, as yet, systematically investigated the relative importance of different types of intertidal vegetation and/or seasonal changes to this bed roughness effect.
This project involved (a) the testing and validating an innovative digital photograph technique for the measurement and parameterisation of saltmarsh vegetation 'roughness'/'density', and (b) the provision of information on the relationships between wave attenuation and vegetation 'roughness'/'density' on a macro-tidal saltmarsh.
Field photography of vegetation and measurements of vegetation parameters and waves were carried out on an open coast marsh near Tillingham, Dengie peninsula, Essex, UK in July 2004, September 2004 and December 2004 at three ca. 10 m long transects (see Plate 1), characterised by different vegetation types. Side-on photographs of vegetation (see Möller et al. (2000) and Zehm et al. (2003) for a description of the methodology) were obtained between the wave recording instruments and in their immediate vicinity and vegetation harvested for determination of wet and dry biomass and species composition / structure.
Plate 1: View to the Northeast across Tillingham marshes from the seawall and (smaller inset) recording Transect 1 (September 2004)
Plate 2 illustrates the process of image classification and vegetation density computation. Wave measurements were collected throughout the tidal inundation period (see also Möller et al. (2001)) and processed to provide wave spectra and wave statistics on the rising tide.
Plate 2: Digitally classified vegetation image and resulting parameters
Result of this work have been published in Möller (2006).
The results from the vegetation monitoring show that the digital photograph method for recording vegetation density variations can be successfully calibrated against more traditional methods of measuring aboveground biomass by harvesting, drying and weighing vegetation. Observed seasonal differences in wave attenuation can be, at least partly, explained by seasonal and spatial differences in vegetation density and canopy height.
To establish the generic nature of these process relationship it is planned to extend the project to longer time-scales (in particular the spring season) and a wider range of vegetation types (e.g. high marsh communities).
- Möller, I. (2006) Quantifying saltmarsh vegetation and its effect on wave height dissipation: results from a UK East coast saltmarsh. Journal of Estuarine, Coastal, and Shelf Sciences, 69(3-4), 337-351
- Möller, I. and Spencer, T. (2003) 'Wave transformations over mudflat and saltmarsh surfaces on the UK East coast - Implications for marsh evolution'. Proc. Int. Conf. on Coastal Sediments '03, Florida, USA.
- Möller, I., Spencer, T., French, J.R., Leggett, D.J., Dixon, M. (2001) 'The sea-defence value of salt marshes - a review in the light of field evidence from North Norfolk'. Journal of the Chartered Institution of Water and Environmental Management 15, 109-116.
- Möller, I., Spencer, T., French, J.R., Leggett, D.J., Dixon, M. (2000). A new perspective on the sea-defence value of salt marshes. Proc. 35th MAFF Conf. of River & Coastal Eng. 5-7 July, Keele, UK, 11.11.1-11.11.4
- Möller, I. (2000) 'Vegetation survey to support wave attenuation monitoring in The Wash'. Report to the Environment Agency, UK.
- Möller, I., Spencer, T., French, J.R., Leggett, D.J., Dixon, M. (1999). Wave transformation over salt marshes: A field and numerical modelling study from North Norfolk, England. Estuarine, Coastal and Shelf Science 49, 411-426.
- Moeller, I., Spencer, T., and French, J.R. (1996) Wind wave attenuation over saltmarsh surfaces: Preliminary results from Norfolk, England. Journal of Coastal Research 12(4), 1009-1016.
- Sutherland, W. J. (1996) Ecological Census Techniques. A Handbook. Cambridge University Press, Cambridge.
- Zehm, A., Nobis, M., and Schwabe, A. (2003) 'Multiparameter analysis of vertical vegetation structure based on digital image processing'. Flora, 198, 142-160.
Conference Presentations / Publications of results to date:
- Oct 2004: 'Quantifying form-process interactions on saltmarshes - a case study from the UK East Coast', Invited Speaker, AGU Chapman Conference on Saltmarsh Geomorphology, Halifax, Canada.
- Möller, I., (in press) 'Quantifying form-process interactions on saltmarshes - a case study from the UK East Coast'. Special Issue, Estuarine, Coastal, and Shelf Sciences.
Funding for this project was obtained through a Royal Geographical Society/EPSRC Geographical Research Grant.